1. Field of the Invention
The present invention relates to a matching circuit for the input and output of a transistor used in high-frequency, high-power amplifier, and more particularly, to a matching circuit for a high-frequency, high-power transistor which is capable of eliminating a reduction of in the amplification efficiency due to a phase difference caused by the spatial dimensions of the transistor, and which is capable of matching the impedance as well.
2. Description of the Prior Art
In the field of electric communications, the signal frequency is becoming higher, and especially in the field of satellite communications, the frequency is exceeding 10 GHz. Along with this trend, the devices and apparatuses used at such frequencies are required to be smaller in size, and accordingly there is an increasing need for inexpensive integrated circuits having favorable characteristics that can be used in the microwave band.
The input and output impedances of high frequency transistors employed in such integrated circuits do not generally coincide with the main transmission line characteristic impedance (50 ohms). In the main transmission line, lines known as microstrip lines are widely employed. In order to amplify an electric signal efficiently, it is desired that the transistor input and output impedances and the impedances of the input and output main line microstrip lines be matched as closely as possible, and that the reflection at the matching point be as small as possible. In particular, the input and output impedances of the high frequency and high-power transistor is much lower than 50 ohms, and usually a low impedance element is inserted parallel to the input and output main line microstrip lines in order to match the impedance. The impedance Zos of an open microstrip line (an open stub) is expressed as follows: EQU Zos=-j.multidot.cot .beta.L (1)
where .beta.=2.pi./.lambda.; .lambda.is the wavelength on the microstrip line at the frequency to be matched; and
L=Length of the microstrip line.
Therefore, Zos becomes smaller as .beta.L approaches .pi./2, that is, as L approaches .lambda./4, and by selecting a proper value, matching with the transistor is achieved.
A typical structure of a conventional high-frequency amplifier according to this method is shown in FIG. 7.
In FIG. 7, numeral 101 denotes a field effect transistor (FET), 102 denotes an input matching circuit substrate, 103 denotes an output matching circuit substrate, 104 is a main line composed of a microstrip line connected to an input terminal, 105 denotes a main line composed of a microstrip line connected to an output terminal, and 106 and 107 denotes so-called taper parts each having a gradually widening electrode width and disposed at the transistor side of the main line. Numerals 110 and 111 denote wires for connecting the transistor and the taper parts, 701 and 702 denote insular electrodes (pads) for the adjustment of input and output impedance matching, respectively, and 703 an 704 denote wires for connecting the taper parts and the adjusting pads. In this construction, the adjustment of the input matching circuit and output matching circuit is achieved by connecting the adjusting pads to the wires. A typical example of such an adjusting method is disclosed in the Japanese Patent Publication 57-23441.
As an improved version thereof, a method of employing chip capacitors for matching is known. For example, a typical example is reported in "Broad-Band Internal Matching of Microwave Power GaAs MESFET's," K. Honjo, Y. Takayama, and A. Higashisaka, IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-27, No. 1, 1979, pp. 3-8.
A typical structure of this method is shown in FIG. 8. In FIG. 8, numerals 101 to 107 denote the same parts as in FIG. 7. Numerals 801 and 802 denote chip capacitors for input and output impedance matching, respectively, and both lower electrodes are connected on a grounded base, and the upper electrodes are connected to the main line microstrip line taper parts of input and output matching adjusting circuit substrates and to the transistor by means of wires 803, 804, 805, 806. In this structure, the input and output matching is achieved by the chip capacitor and the inductance of the wire connecting it.
Further, a method of matching by using a thin-film capacitor instead of the chip capacitor is disclosed in "Microwave Integrated-Circuit Technology-A Survey," M. Caulton, and H. Sobol, IEEE Journal of Solid-State Circuits, Vol. SC-5, No. 6, 1970, pp. 292-303.
In these conventional methods, however, matching of only the impedance is taken into consideration, and no consideration is given to the phase difference of electric signals in the taper parts. Moreover, such methods are insufficient to realize matching circuits for a high-frequency, high-power FET having a gate width comparable to the signal wavelength, in particular. At 14 GHz, for example, the length corresponding to 1/4 wavelength on the alumina substrate or GaAs substrate is about 2 mm. On the other hand, the gate width of the GaAs FET for obtaining an output of 3 watts is about 4 mm. Therefore, there is a considerable phase difference between the electric signal passing the central part of the taper part and the electric signal passing the end part. When a phase difference occurs in the input signal, a phase difference also takes place in the signal after being amplified by the FET, and as a result the synthesized output signal is attenuated, and the amplification efficiency is lowered. At the taper part in the output area, a spatial phase difference also occurs, and the performance is further lowered.
In the matching method by the open stub shown in the first prior art, it is considerably difficult to match the high-frequency, high-power FET which has low input, output impedances, and usually the composition of the second prior art is employed.
In the case of the second prior art, however, it is necessary to connect a large chip capacitor separately. Accordingly, it is easier to match the impedance than in the first prior art, but in the manufacturing procedure the process for mounting the chip is increased, and a chip mounting part is additionally required, which makes it hard to reduce the size and integrate to a high degree. As a result the manufacturing cost becomes higher.
By modifying the shape of the taper parts to reduce the spatial phase difference, other methods are proposed for example in the Japanese Patent Publications 64-50602, 64-74812, but these are not intended to satisfy the impedance matching simultaneously.
Incidentally, as a method of matching while eliminating the spatial phase difference, so-called power distributors and power synthesizers using 1/4 wavelength impedance converters are known, and they are generally used in the power amplifiers of several watts or more. It is, however, difficult to reduce the size thereof because an impedance converter in the length of at least 1/4 wavelength is required.